1. Field of the Invention
The present invention relates to a power supply device with fast output voltage switching capability, and more particularly, to a power supply device with fast output voltage switching capability and lower power consumption.
2. Description of the Prior Art
For reducing production cost, few voltage supplies are employed in electronic devices. Thus, when the circuit is in need of various DC voltages, a DC-DC converter is used to perform such voltage conversions, so the number of external components and integrated circuits can be saved.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a conventional DC-DC boost converter 10. In order to make an output voltage VOUT stable, the DC-DC boost converter 10 generally utilizes a Pulse Width Modulation (PWM) controller 11 to generate a switch signal that controls a duty cycle of a power transistor 102 for increasing or decreasing energy delivered to load elements. Detailed operations of the DC-DC boost converter 10 is well known by those skilled in the art, and thus not described particularly herein. In the prior art, when the output voltage VOUT is switched from high to low, the DC-DC boost converter 10 may temporarily cease operation of the PWM controller 11 such as by disabling the switch signal, for example, to allow a load current Iload extracting charges from a load capacitor CL, so as to level down the output voltage VOUT. Until the output voltage VOUT descends to a target level, the operation of the PWM controller 11 is then resumed.
In such a case, the descending speed of the output voltage VOUT would be affected by sizes of the load capacitor CL. Moreover, a great amount of energy may be unnecessarily consumed by the external circuits, especially for some specific applications of the DC-DC boost converter 10 that need to connect with large external capacitors such as an LED (Light Emit Diode) backlight module of a liquid crystal display (LCD), for example.
Besides, the prior art can further create additional current paths to accelerate voltage switching of the output voltage. For example, please refer to FIG. 2, which is a schematic diagram of a conventional DC-DC buck converter 20. In this case, when the output voltage VOUT is switched from high to low, the DC-DC buck converter 20 turns on a switch SL to allow charges of the load capacitor CL being directly transferred to the ground via an inductor L1, so the output voltage VOUT can be lowered rapidly. However, the way to directly transfer the charges of the load capacitor CL to the ground is kind of wasting of the energy. On the other hand, when the output voltage VOUT is switched from low to high, the DC-DC buck converter 20 then turns on a switch SH to allow an input voltage Vin directly charging the load capacitor CL via the inductor L1, so the output voltage VOUT can be raised rapidly. However, the way to raise the output voltage VOUT is not suitable for use when the output voltage VOUT is greater than the input voltage Vin, i.e., not suitable for the DC-DC boost converter.
Thus, even though the prior art can provide additional current paths to accelerate the voltage switching of the DC-DC converter, it may still suffer waste of energy, and cannot be applied to the DC-DC boost converter.